Cryogenic valves are generally designed to function in the extremely cold temperatures of cryogenic fluids, such as liquid nitrogen. Most components of such valves are fabricated from stainless steel. It is common to find cryogenic valves designed with features that protect their components from the effects of the fluid. For example, some cryogenic valves have stem packing that is positioned beyond a point of insulation. Another example is the use of an extended valve bonnet that enables thermal conduction; cryogenic fluid that might escape into the bonnet is thus able to warm and vaporize. Such features can enhance the reliability of valve components.
However, such features can also introduce design challenges. The extended bonnets common in cryogenic valves house valve stems or spindles, which operably engage with some form of valve actuator and the fluid sealing portion of the valve. Within the bonnet, the stem or spindle is often maintained in position by a guide bushing. Like the bonnet, the spindle is often extended. Commonly, the material polytetrafluroethylene (PTFE) is used for guide bushings; the guide bushing controls the extended spindle for proper alignment, which aids in the proper operation and sealing of the valve. PTFE has a low coefficient of friction, and is generally both tough and suitable for use at cryogenic temperatures, rendering it an industry standard material for such purposes.
Cryogenic valves are commonly rated for a temperature range between the desired temperatures of the cryogenic fluid to that of ambient temperature. For an application or use limited to liquid nitrogen, the cryogenic fluid system may experience a change or delta in temperature of around 350 degrees Fahrenheit.
Industrial applications for fluid cryogenic systems include diverse fields such as power generation, fuels, food service, research, health applications (magnetic resonance imaging), etc. A subset of these applications requires, at different times, the use of both steam and cryogenic fluid within the same fluid system. For example, both steam and cryogenic fluids are highly used in the food industry for the rapid heating and cooling of food. For a liquid nitrogen and steam application, the fluid system may experience a change or delta in temperature of around 700 degrees Fahrenheit.
PTFE (e.g., appr. 100×10−6 mm/mm*C) has a larger coefficient of thermal expansion than austenitic stainless steel (e.g., appr. 18×10−6 mm/mm*C), though actual values vary by composition and temperature range of measurement. Some PTFE may experience a linear thermal expansion on the order of 5% over a large delta increase or range of temperature. According to some suppliers, PTFE exhibits a significant change in a critical transition zone of 65-77 degrees Fahrenheit, with a volumetric change there of about 1.0-1.8% alone. For operating conditions exclusively on either side of this transition zone, precision parts may be produced while under the temperature conditions similar to that of use to avoid such a change. However, a broad range of 700 degrees Fahrenheit overlapping both sides of the transition zone renders volumetric changes unavoidable.
A PTFE component will generally shrink or expand to greater extent than a stainless steel component. In a conventional cryogenic valve, the greater shrinking in extremely low temperatures and expansion at higher, ambient temperatures may be accommodated, but may not be ideal.
In a cryogenic valve subject to the temperatures of cryogenic fluid and steam, the thermal expansion characteristics may be too severe. For example, as a bushing within a cryogenic valve, the tolerances of the finished part vary far too much during that temperature range to maintain proper alignment of the plug and valve seat. In practice, the use of conventional PTFE bushings over such a broad range of temperature has resulted in broken stem or spindle assemblies. The PTFE bushing fabricated to tolerances for proper alignment will expand to such an extent as to encounter the lesser expanding stainless steel valve body.
A device or design permitting the use of precision parts in such steam and cryogenic fluid systems would desirably overcome this problem. A material matching the thermal expansion characteristics of austenitic stainless steel would ideally solve many of the problems of PTFE bushings. A stainless steel bushing on a stainless steel bonnet generally creates too much friction which can lead to galling of the two components. As a result, improvements in the field are desired.